This invention relates to a printer in which magnetic energy accumulated in a drive coil in a carriage section is released to a control unit, and more particularly to protection for such a printer from overheating and damage to the components due to disconnection of a discharge line in the carriage section from the control unit.
A serial printer is known in which a carriage section moving in the direction of printing (direction of spacing), and a control unit for controlling the printing operation of the carriage section are in the form of units separate from each other and these are connected together by a cable. In such a serial printer, a power supply circuit for supplying drive power to a print head mounted on the carriage section is disposed in the control unit, and the power is supplied through the cable to the carriage.
FIG. 4 shows part of the prior art printer driver, including the power supply nodes or lines. Connected to the positive electrode Vp of the driver power supply circuit 7 disposed in the control unit 1 are a drive power supply line 38a and a discharge line 37a. Connected to the negative electrode Ep is a ground line 39a. The lines 37a, 38a and 39a are connected to respective terminals in an output port, and via a cable 3 to a discharge line 37b, a drive power supply line 38b and a ground line 39b disposed in a metal carriage section 4. Moreover, a switching element 16, formed for example of a transistor, is interposed in the drive power supply 38a, and the drive power supply line 38a and the ground line 39a are shunted via a series connection of diodes 17a, 17b and 17c.
In the metal carriage section 4, dot wire drive coils 15a, 15b, etc. are provided in the print head 6. One end of each of the coils 15a, 15b, etc. is connected to the drive power supply line 38b, while the other end is connected to the ground line 39b via switching elements in a switching circuit 12. Accordingly, formed between the drive power supply circuit 7 and the coils 15a, 15b, etc. is a drive power supply path comprising the switching element 16, the drive power lines 38a, 38b, the cable 3, the switching circuit 12, and the ground lines 39a, 39b. Drive power is supplied to the coils that are selected in accordance with the print data. For instance, when the coil 15a is selected, a drive current I.sub.A flows through the coil 15a as shown in the equivalent circuit of FIG. 5A. This current flows through the drive power supply 7 in the forward direction. Because of this current, the dot wire (not shown) in the print head is driven forward toward the print medium on a platen (not shown), and printing is performed.
Thus, when the drive current I.sub.A flows, magnetic energy is accumulated in the coil 15a. When the switching element 16 is opened and the current path from the power supply circuit 7 is interrupted, the accumulated energy acts to maintain the current I.sub.B through the coil 15a. This current flows through a closed path including the coil 15a, the switching element 12, and the diodes 17c, 17b and 17a. The coil 15a thereby maintains the electromagnetic force. The dot wire is kept in the projecting state for the period while the current I.sub.B flows.
When the switch 12 is opened, as shown in FIG. 5C, an energy discharge current I.sub.C flows through the path including the diodes 17c, 17b and 17a, the coil 15a, and the diode 13a. This current flows through the drive power supply 7 in the reverse direction. In other words, this current is a regenerative current due to an electromotive force induced in the coil 15a attempting to maintain the current in the same direction (when it passes through the coil) as before the current path is changed. The coil 15a thereby loses the electromagnetic force and the dot wire of the print head returns to the original position, and is ready for next printing stroke. The current which flows through the coil assumes a waveform as shown by solid line A in FIG. 6.
When the control unit 1 and the metal carriage section 4 are connected together by a cable 3 as described above, it many happen that the joint, such as a connector, between the cable 3 and the control unit 1, or the joint, such as a connector, between the cable 3 and the metal carriage section 4 has an unsatisfactory connection. The unsatisfactory connection may be either a total disconnection or a poor connection. When the connection is unsatisfactory, the printer may operate erroneously or may break down. To prevent this, a limit switch may be provided to supervise the insertion of the connector and hence the connection. The limit switch and outputs a signal to inhibit the printing operation when the connection is improper. With the conventional detecting device, however, it was not possible to detect unsatisfactory connection of individual conductors in the cable 3. When, therefore, a certain conductor was broken or poorly connected, this was not detected and the connection waws considered proper as long as the insertion of the connector was proper.
If any of the conductors forming the drive power supply lines 38a and 38b, and ground lines 39a and 39b is broken or poorly connected, power is not supplied to the print head 6, and the printing is not initiated. If a logic power supply line, data signal line, enable signal line or the like is broken or poorly connected, the printing is disabled, and the printing is not started. The user or the operator of the printer is therefore immediately informed of the abnormality without damage (degradation or breakdown) to the circuit components.
However if the discharge line 37a or 37b is broken or poorly connected, printing is not disabled, and there appears to be no abnormality. The operator will not notice the broken or poorly connected line. However, in such a case, as shown by the dotted line in FIG. 6, the energy accumulated in the coils 15a or 15b is not fully discharged before the next cycle of operation. The impedance of the coil is therefore effectively reduced, and the coil current in the next cycle is increased, and the heat generation in the metal carriage section 4 is increased. This may cause degradation or breakdown of circuit components.
Moreover, because the drop in the coil current is not sharp, the return of the dot wire is not swift, and the printing quality is degraded.